The invention relates, in general, to coherent radio frequency memories (CRFM) and, more specifically, to digital radio frequency memories (DRFM) suitable for use in military radar countermeasure equipment.
Active radar jammers are used in the field of electronic countermeasures to confuse or counter a system originating radar signals. In some situations, it is desirable to return signals to the radar system which are exact copies of the arriving radar signal. In other situations, it is desirable to return signals to the radar system which have characteristics other than that of the received radar signal in order to further confuse the radar system. In any event, it is usually necessary for the countermeasure system to store the received radar signal and to reproduce it at a later time.
Previously, delay lines of various types have been used effectively to store the received radar signal for a short period of time and make the stored radar signal available at a later time. One of the disadvantages of delay lines is that the delay cannot be electronically changed easily. Also, it is difficult to obtain reasonably long delay periods without serious signal degradation. Also, delay lines are usually large and bulky.
An improvement over the delay line technology has been achieved by the use of digital radio frequency memories (DRFMs) which convert relatively high radio frequency (RF) signals down to a low intermediate frequency (IF) frequency by mixing the RF with a local oscillator (LO) signal. The IF signal is then digitized for storage in a digital memory device. The digital memory can be controlled in a manner similar to that of a digital memory of a computer and the stored value representing the radar signal can be recalled and replicated at any time delay desired. Further, the manipulation of the digital values to produce changes in the replicated signal are also conveniently done by the digital process.
U.S. Pat. No. 4,713,662 entitled Modulated Digital Radio Frequency Memory, in the name of Richard J. Wiegand, the inventor herein, and assigned to Westinghouse Electric Corporation, the assignee herein, discloses a variety of DRFMs including a two-channel or I&Q system as well as a modulated single channel DRFM. The I&Q or two-channel DRFM stores an RF pulse by preserving the frequency and phase of an RF input local oscillator and a sampling clock while the remainder of information is held in digital logic chips such as a RAM. In the modulated single channel DRFM in addition to preserving frequency and phase of the RF local oscillator signal, a modulation pattern for the RF local oscillator is preserved and applied to the input and output.
The DRFMs described in Wiegand's patent employ one bit sampling, that is, the IF input is sampled to determine if the signal is positive or negative at each sample point. One bit sampling reduces RAM size to a minimum while allowing maximum sampling frequency and maximum amplitude dynamic range. However, the stored DRFM signal is quite non-linear resulting in numerous spectral lines or spurs in addition to the main desired coherent spectral lines. Also, an image of the input may be generated which is undesirable. While various techniques are available to significantly reduce spurs in both the I&Q and single channel DRFMs further improvement is desirable.